In 3G, both HNBGW and GGSN talk GTPv1U, and osmo-sgsn uses Direct Tunnel feature to connect both together. Since GTPv1U uses a fixed port, we need 2 IP addresses, one set for the emulated GGSN and one for the emulated HNBGW.
This is a preparation towards adding tests validating creation of PDP context in 3G, which was not yet tested.
